This invention generally relates to dry etching methods, and it particularly pertains to a dry etching method for selectively dry etching a silicon nitride layer which is formed through an intermediate, underlying layer such as a silicon oxide layer (SiO.sub.2) on the surface of a semiconductor substrate.
Conventional dry etching methods have employed a carbon-containing gas as a main etching gas such as CF.sub.4, CH.sub.2 F.sub.2, and CH.sub.3 F, in selectively etching a silicon nitride layer formed on the surface of an underlying SiO.sub.2 layer as well as in at the same time ensuring etch-selectivity to the SiO.sub.2 layer (International Electron Devices Meeting 1983, Technical Digest, pp. 757-759, by T. Kure et al.).
Such a carbon-containing main etching gas, however, presents a problem. The presence of carbon in a main etching gas accelerates the reaction of the main etching gas and the SiO.sub.2 layer. This results in the decline of etch-selectivity, that is, the ratio of the etch rates of the silicon nitride layer and the SiO.sub.2 layer.
Japanese Pat. Appln. published under No. 2-262334 proposes a technique that uses a mixed gas of NF.sub.3 and Cl.sub.2 as a main etching gas. Japanese Pat. Appln. published under No. 2-66943 shows another technique that uses ClF.sub.3 as a main etching gas. The use of such main etching gases without containing carbon has the effect of controlling the etching of a SiO.sub.2 layer, which is detailed in Japanese Pat. Appln. published under No. 3-22532.
None of these main etching gases, however, contain a CH group, so that no passivation films, of polymer, for protecting an etching wall are formed. As a result, the amount of CD loss (i.e., the value obtained by a subtraction of the resist pattern size prior to an etching process minus the silicon nitride layer pattern size after the etching process) becomes roughly equal to the thickness of the silicon nitride layer, 0.1-0.2 .mu.m. They are therefore not a suitable main etching gas for silicon nitride layers of semiconductor memories not less than 16M DRAM.
As a solution to the above problem, Japanese Pat. Appln. published under No. 2-271614 discloses a method of using a mixed gas of NF.sub.3 and HBr, and Japanese Pat. Appln. published under No. 3-22532 shows another method of using a mixed gas of HBr and O.sub.2.
In accordance with these methods, SiBr.sub.4 with a less volatility, produced by the reaction of HBr in the mixed gas and Si contained in a Silicon nitride layer of a semiconductor substrate, is forced to deposit on an etching sidewall to protect it, as in a phenomenon taking place at the time of etching to a polysilicon layer described in U.S. Pat. No. 4,490,209. Thus an anisotropic etching can be realized thanks to the protection of an etching sidewall.
For the etching of a silicon nitride layer with HBr, a reaction product, SiBr.sub.4 will deposit on the walls of a reaction chamber, as in the etching of a polysilicon layer with HBr. Therefore, the deposits on the walls peel off to form dust particles. This presents another problem that the yield of semiconductor device declines.
An effective method to control the deposition of SiBr.sub.4 on the walls of a reaction chamber is known, which controls the temperature of a reaction chamber wall in the 50.degree.-100.degree. C. range. It is however not practical at all to keep the entire wall at such a high temperature range. For this reason, when etching a silicon nitride layer with HBr, it is not possible to control the generation of dust particles.